1. Field of the Invention
The present invention relates to a method of reducing leakage current of a photodiode, and more particularly, to a method of reducing leakage current of a photosensing area of a photodiode.
2. Description of the Prior Art
A photodiode is a semiconductor device comprising a photoconductivity cell and a junction diode, and is commonly used in photoelectric products, such as cameras and photosensors of scanners. A light-induced current of the photodiode represents a signal, whereas a current present in an absence of light represents noise. The photodiode processes signal data by using a magnitude of a signal-to-noise ratio. In the semiconductor industry, it is often desirable to increase the light-induced current of the photodiode so as to increase the signal-to-noise ratio, and hence to enhance a contrast of the signal. A sensitivity of the photodiode is enhanced and a quality of the photodiode is improved.
Please refer to FIG. 1. FIG. 1 is a cross-sectional diagram of a structure of a photodiode 30 according to the prior art. The semiconductor wafer 10 comprises a silicon substrate 12, and a p-well 14 positioned on the silicon substrate 12. The photodiode 30 comprises a photosensing area 16 positioned on the surface of the p-well 14. Additionally, the semiconductor wafer 10 comprises an insulation layer 18 positioned on the surface of the silicon substrate 12. The insulation layer 18 surrounds the photosensing area 16 and serves as an insulating material to prevent short-circuiting between the photosensing area 16 and other units.
According to the prior art method of forming the photosensing area 16 of the photodiode 30, an ion implantation process is first performed to form an n-type doped region 20 on the surface of the p-well 14. Arsenic (As), with an energy of about 80 KeV and a dosage about 1015 ion/cm2, is used as a major dopant in the ion implantation process. A depletion region 22 for detecting the leakage current is formed along the PN junction between the doped region 20 and the adjacent p-type well 14. In FIG. 1, the area marked with slanting lines illustrates the depletion region 22.
In the formation of the photodiode 30 according to the prior art, dopants with an energy of about 80 KeV used in the ion implantation process not only create a deeper PN junction depth, but also damage a crystal structure on a surface of the photosensing area 16. Thus, more dark current (that is, current in the absence of light) is generated at the interface between the p-well 14 and the doped region 20 below the insulation layer 18. As a result, the signal-to-noise ratio and the sensitivity of the photodiode 30 are reduced.
It is therefore a primary objective of the present invention to provide a method of reducing leakage current of a photodiode that increases signal-to-noise ratio and sensitivity.
The present invention provides a method of reducing leakage current of a photodiode in a semiconductor wafer. The semiconductor wafer comprises a p-type substrate, a photosensing area for forming a photosensor of the photodiode, and at least one shallow trench surrounding the photosensing area to prevent short-circuiting between the photosensing area and other units. The method of the present invention is forming a doped polysilicon layer containing p-type dopants in the shallow trench. Then a thermal process is performed to cause the p-type dopant in the doped polysilicon layer to diffuse into the p-type substrate to form a p-type doped region surrounding a bottom of the shallow trench and walls of the shallow trench. After that, the doped polysilicon layer is removed and an insulator material is filled into the shallow trench to form a shallow trench isolation (STI) structure. A first n-type implantation process is performed with arsenic or phosphorus ions to form a first n-type doped region in the photosensing area of the photodiode. Then a second n-type implantation process is performed with the arsenic or phosphorus ions to form a second n-type doped region in the photosensing area of the photodiode. Here, the p-type doped region surrounding the trench is used to decrease the electric field in the interface between the photosensing area and the insulation layer to reduce the leakage current.
It is an advantage of the present invention that a p-type doped region is formed at the interface between the insulation layer and the photosensing area to reduce leakage current efficiently and enhance the performance of the photodiode.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.